In recent years, light-emitting element such as organic EL lighting has been drawing attention as an eco-friendly green device. Features of the organic EL lighting include: 1) a low power consumption as compared with an incandescent lamp; 2) a thin profile and light weight; and 3) a flexibility. At the moment, the organic EL lightings are being developed so as to attain the above features of 2) and 3). In this respect, glass substrates that have been conventionally used in flat panel displays (FPD) and the like are unable to attain the above features of 2) and 3).
In view of this, there are researches being conducted on a substrate as a support (hereinafter, “supporting base material”) for organic EL lighting, proposing an ultra-thin glass, a resin film, a metal foil or the like as a possible supporting base material. The ultra-thin glass is superior in heat resistance, barrier performance and optical transparency and has good flexibility, but is somewhat inferior in handling and has low thermal conductivity and high material cost. Further, the resin film is superior in handling and flexibility and has low material cost and good optical transparency, but is inferior in heat resistance and barrier performance and has low thermal conductivity.
In contrast, apart from the absence of optical transparency, the metal foil has excellent features such that it is superior in heat resistance, barrier performance, handling, and thermal conductivity and also has good flexibility and low material cost. In particular, a typical flexible glass or film has an extremely low thermal conductivity of 1 W/m° C. or lower, while a copper foil has an extremely high thermal conductivity of around 280 W/m° C.
In order to attain a light-emitting element using a metal substrate, Patent Literature 1 (JP2009-152113A) proposes that a surface of the metal substrate is smoothed by polishing treatment and plating treatment, and that thereon an organic layer is formed. In addition, Patent Literature 2 (JP2008-243772A) proposes that by providing a nickel plated layer on the metal substrate, a smooth surface is formed without polishing and the like, and that thereon an organic EL element is formed. On the other hand, a photo-electric element using a metal substrate is proposed, for example, Patent Literature 3 (JP2011-222819A) discloses a solar cell with an organic thin film electromotive force layer provided on the smoothing-treated metal base material. In these techniques, for prevention of short circuit between electrodes, it is an important subject to smooth the surface of the metal substrate. As techniques that deal with the subject, Patent Literature 4 (WO2011/152091) and Patent Literature 5 (WO2011/152092) propose that a metal foil comprising an ultra-smooth surface, which has an extremely low arithmetic average roughness Ra of 10.0 nm or less, is used as both a supporting base material and an electrode.
Turning to the light-emitting element and the photo-electric element, it is known to be advantageous to scatter light by the surface of the metal foil, in order to bring out light or to take in light efficiently. Namely, high light scattering results in an increase in light-emitting efficiency in the light-emitting element, and thus increases power generation efficiency in the photo-electric element. In this respect, since it is difficult to sufficiently scatter light without resulting in short circuit at the surface of the conventional metal foil, in the case of the light-emitting element, an attempt is made to improve light-emitting efficiency by disposing microlens or the like on the light-takeoff side of the substrate surface. However, formation of the microlens or the like is not advantageous from the viewpoint of productivity and cost due to the necessity of microfabrication, and therefore it is impossible to say that the disposition of the microlens is suitable for widening of area.